Unusual two switch setup

[conroyster]

Thanks for the reply. Apologies if I am a little vague, but I don't have ready access to the setup, therefore I cannot elaborate on it further.

As mentioned previously, what struck me was that the switch in the hall has a single red wire and a single black wire, which, from my limited knowledge, I thought could only exist in a 1-way switch.

 

[ColJack]

Borrowed neutrals do not present a hazard to those using the installation but they do present a hazard to those maintaining and/or modifying it.

When you disconnect the neutral to a load while leaving the live connected the disconnected neutral conductor will become live. While it will have a much higher impedance than a supply conductor it's impedance will still be more than low enough to deliver a fatal shock.

Now consider an installation with a borrowed neutral. Consider the situation where you are working on the circuit the neutral is borrowed from before the place the neutral is borrowed. You isolate the circuit and confirm it is dead, then you start disconnecting a loop-in wired fitting to replace it and in doing so break the neutral conductor to the fitting with a borrowed neutral. The neutral conductor to the next fitting in the circuit is now live.

all very true points, but you shouldn't be working on live installations.. the power should be off..
so unless you happen to live at one of the poles, or an extremely southerly or northerly town, then you do lights during daylight hours and have no excuse for not isolating the entire house..

 

[JohnnyG]

I was reading this the other day about 'borrowed' neutrals, the fatality case quoted highlights that it's always good practice to fully isolate if and when possible.

 

[RF Lighting]

Borrowed neutrals do not present a hazard to those using the installation but they do present a hazard to those maintaining and/or modifying it.

When you disconnect the neutral to a load while leaving the live connected the disconnected neutral conductor will become live. While it will have a much higher impedance than a supply conductor it's impedance will still be more than low enough to deliver a fatal shock.

Now consider an installation with a borrowed neutral. Consider the situation where you are working on the circuit the neutral is borrowed from before the place the neutral is borrowed. You isolate the circuit and confirm it is dead, then you start disconnecting a loop-in wired fitting to replace it and in doing so break the neutral conductor to the fitting with a borrowed neutral. The neutral conductor to the next fitting in the circuit is now live.

It is unlikely in a domestic situation, but the worst scenario is where the live is fed from one CU, and the neutral borrowed from another.

If you switch off the main switch on the CU which has the neutral borrowed from it, every live and neutral wire on that part of the installation will rise to mains potential, yet the installation will appear that it is properly isolated as none of the lights / appliances will be working.